Reducing MEMS stiction by deposition of nanoclusters

ABSTRACT

A mechanism for reducing stiction in a MEMS device by decreasing surface area between two surfaces that can come into close contact is provided. Reduction in contact surface area is achieved by increasing surface roughness of one or both of the surfaces. The increased roughness is provided by forming a micro-masking layer on a sacrificial layer used in formation of the MEMS device, and then etching the surface of the sacrificial layer. The micro-masking layer can be formed using nanoclusters. When a next portion of the MEMS device is formed on the sacrificial layer, this portion will take on the roughness characteristics imparted on the sacrificial layer by the etch process. The rougher surface decreases the surface area available for contact in the MEMS device and, in turn, decreases the area through which stiction can be imparted.

BACKGROUND

1. Field

This disclosure relates generally to manufacture ofmicroelectromechanical systems, and more specifically, to reducingstiction in MEMS devices through the use of nanoclusters as amicro-masking layer.

2. Related Art

Microelectromechanical systems (MEMS) devices are micromechanicaldevices that provide moving parts having features with dimensions below100 μm. These moving parts are formed using micro-fabricationtechniques. MEMS devices have holes, cavities, channels, cantilevers,membranes, and the like. These devices are typically based on siliconmaterials and use a variety of techniques to form the proper physicalstructures and to free the mechanical structures for movement.

Stiction is a static friction force that is a recurrent problem withtypical MEMS devices. While any solid objects pressing against eachother without sliding require some threshold of force (stiction) toovercome static cohesion, mechanisms generating this force are differentfor MEMS devices. When two surfaces with areas below the micrometerrange come into close proximity, the surfaces may adhere together due toelectrostatic and/or Van der Waals forces. Stiction forces at this scalecan also be associated with hydrogen bonding or residual contaminationon the surfaces.

For MEMS devices such as accelerometers, surfaces such as over-travelstops come into close proximity or contact during use at the limits ofthe device design. In those situations, stiction forces can cause theMEMS device parts (e.g., a teeter-totter accelerometer mechanism) tofreeze in place and become unusable. Traditional methods of avoidingsuch close proximity travel or contact include increasing springconstants and increasing distance between parts of the MEMS device. Butcompensating for stiction in this manner can decrease sensitivity of thedevice, and therefore decrease the utility of the MEMS device. It istherefore desirable to provide a mechanism for reducing stiction-relatedinteractions of MEMS devices without also decreasing sensitivity of theMEMS device.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart by referencing the accompanying drawings.

FIG. 1 is a simplified block diagram illustrating a cross section viewof an accelerometer known in the prior art.

FIG. 2 is a simplified block diagram illustrating a close up of a crosssection view of the travel stop region at an end of a MEMS accelerometerduring a stage of fabrication.

FIG. 3 is a simplified block diagram illustrating a cross-section viewof the travel stop region during a stage of fabrication subsequent tothat of FIG. 2.

FIG. 4 is a simplified block diagram illustrating a cross-section viewof the travel stop region during use of the accelerometer.

FIG. 5 is a simplified block diagram illustrating a cross-section viewof the travel stop region during a stage of fabrication performed inaccord with embodiments of the present invention.

FIG. 6 is a simplified block diagram illustrating a cross-section viewof the travel stop region during a stage of fabrication subsequent tothat illustrated in FIG. 5.

FIG. 7 is a simplified block diagram illustrating a cross-section viewof the travel stop region during a stage of fabrication subsequent tothat illustrated in FIG. 6.

FIG. 8 is a simplified block diagram illustrating a cross-section viewof the travel stop region during a stage of fabrication subsequent tothat illustrated in FIG. 7.

FIG. 9 is a simplified block diagram illustrating a cross-section viewof the travel stop region subsequent to removal of a sacrificial layer,where the travel stop is formed in accord with embodiments of thepresent invention.

The use of the same reference symbols in different drawings indicatesidentical items unless otherwise noted. The figures are not necessarilydrawn to scale.

DETAILED DESCRIPTION

Embodiments of the present invention provide a mechanism for reducingstiction in a MEMS device by decreasing surface area between twosurfaces that can come into close contact. Reduction in surface area isachieved by increasing surface roughness of one or both of the surfaces.Embodiments provide such increased roughness by forming a micro-maskinglayer on a sacrificial layer used in formation of the MEMS device, andthen etching the surface of the sacrificial layer. The micro-maskinglayer can be formed using nanoclusters (e.g., poly silicon or polygermanium). When a next portion of the MEMS device is formed on thesacrificial layer, this portion will take on the roughnesscharacteristics imparted on the sacrificial layer by the etch process.The rougher surface decreases the surface area available for contact inthe MEMS device and, in turn, decreases the area on which stiction canbe imparted.

FIG. 1 is a simplified block diagram illustrating a cross section viewof a teeter totter accelerometer as known in the art. Accelerometer 100includes a substrate 110 with an insulating layer 120. Substrate 110 canbe, for example, a silicon wafer and insulating layer 120 can be, forexample, a silicon oxide or silicon nitride. In some cases, insulatinglayer 120 can be thermally grown from substrate 110 or the insulatinglayer can be deposited.

Fixed electrodes 130 and 135 are formed on top of insulating layer 120,along with travel stop regions 140 and 145. The layer forming fixedelectrodes 130 and 135 and travel stop regions 140 and 145 is typicallypolysilicon and is formed using conventional techniques, includingpatterning the layer as desired for the application. The layer formingthe fixed electrodes and travel stop regions can also be amorphoussilicon, a nitride, a metal-containing material, another suitablematerial, and the like, or any combination thereof. A dielectric layer150 is formed to electrically isolate the electrodes and travel stopregions from other elements of the MEMS accelerometer. Dielectric layer150 can be formed from a variety of materials, including, for example,silicon nitride, silicon dioxide, silicon oxynitride, and the like.

A pivoting proof mass 160 is configured to move in a manner similar tothat of a teeter totter upon acceleration. Pivoting proof mass 160 canbe configured in a manner such that there is an imbalance between a side170 of the pivoting proof mass and side 175 of the pivoting proof massthrough pivot point 165. The amount of imbalance will have an effect ofmaking the device more or less sensitive to acceleration. An electrode180 configured on side 170 of the pivoting proof mass is associated withfixed electrode 130, while an electrode 185 on the pivoting proof massis associated with fixed electrode 135. In addition, a travel stop 190on side 170 of the pivoting proof mass is associated with travel stopregion 140 and a travel stop 195 on side 175 of the pivoting proof massis associated with travel stop region 145. Pivoting proof mass 160,including travel stops 190 and 195 are typically formed of polysilicon.

Electrode 180 and fixed electrode 130 form a first variable sensecapacitor, while electrode 185 and fixed electrode 135 form a secondvariable sense capacitor. Changes in the capacitances of the first andsecond variable sense capacitors can be combined to provide adifferential output from MEMS accelerometer 100.

Fabrication of the components of MEMS accelerometer 100 can be performedusing known MEMS fabrication processes.

FIG. 2 is a simplified block diagram illustrating a close up of a crosssection view of the travel stop region at end 175 of MEMS accelerometer100 during a stage of fabrication. As discussed above, a substrate 110is provided with insulating layer 120, where substrate 110 can be asilicon wafer and insulating layer 120 can be a silicon oxide. A firstpolysilicon layer 210 is formed on insulating layer 120, forming, inpart, travel stop region 145. Dielectric layer 150 is formed overinsulating layer 120 and polysilicon layer 210, in order to, forexample, prevent excessive etching of insulating layer 120. Asacrificial layer 220 is formed on top of patterned dielectric layer 150and exposed regions of polysilicon layer 210. Sacrificial layer 220 iscommonly formed using tetraethyl orthosilicate (TEOS) gas to form asacrificial layer of silicon oxide or the sacrificial layer can beformed of phosphosilicate glass (PSG). The sacrificial layer can bepatterned to form a “mold” for the next layer of the MEMS device. Asecond polysilicon layer 230 can be formed on the patterned sacrificiallayer to form pivoting proof mass 160, including travel stop 195. Thebuildup of patterned layers can continue as needed for the application.

FIG. 3 is a simplified block diagram illustrating a cross-section viewof the travel stop region during a stage of fabrication subsequent tothat of FIG. 2. Sacrificial layer 220 is commonly removed using anisotropic wet etch process selective to the sacrificial layer. Such anetch is performed by either a vapor or a liquid phase process. Butcapillary forces due to surface tension of the liquid used for theetching process or liquid byproducts of the etch process, between travelstop 195 and polysilicon travel stop region 145, as illustrated bymeniscus 310, can cause the surfaces to adhere together during drying.Separating the two surfaces is complicated due to the fragile nature ofthe microstructure. Use of travel stops helps to minimize contactbetween surfaces in a MEMS device, which can help to reduce adhesionduring drying. But this does not necessarily eliminate the problem, asthe surface of the travel stop is subject to stiction as well.

FIG. 4 is a simplified block diagram illustrating a cross-section viewof the travel stop region during a use of the accelerometer.Acceleration A on pivoting proof mass 160 is sufficient to exceed thedesign specification for the accelerometer. This causes travel stop 195to impact travel stop region 145, thereby preventing contact ofelectrode 185 to fixed electrode 135. In this case, however, stictionforces such as Van der Waals, electrostatic, and/or hydrogen bonding cancause the travel stop surface to stick to the travel stop region. It isdesirable to prevent stiction in both the scenarios illustrated in FIG.3 and FIG. 4.

Embodiments of the present invention provide a mechanism by which thesurface area of travel stop 195 that can come in contact with travelstop region 145 is reduced. As discussed above, the reduced surface areadecreases the magnitude of the stiction forces. This is accomplished byroughing up at least the contact surface of travel stop 195 by modifyingthe surface of sacrificial layer 220 at least in the region in whichtravel stop 195 is formed.

FIG. 5 is a simplified block diagram illustrating a cross-section viewof the travel stop region during a stage of fabrication performed inaccord with embodiments of the present invention. As described abovewith regard to FIG. 2, sacrificial layer 220 is formed above depositedand patterned polysilicon layer 210 and dielectric layer 150. A region510 is formed by patterning and etching the sacrificial layer, andcorresponds to travel stop 195. Subsequent to the forming and patterningof sacrificial layer 220, nanoclusters 520 are formed on the surface ofsacrificial layer 220. In one embodiment of the present invention,nanoclusters 520 are formed using methods known in the art ofnanocrystalline silicon. For example, a low temperature, low pressurechemical vapor deposition process can be used to partially depositpolycrystalline silicon on the available surface of sacrificial layer220. An anneal process is then performed, which causes the depositedsilicon to cluster and form spherical or hemispherical regions ofsilicon that are called nanoclusters. Alternatively, low temperature,low pressure deposition of polycrystalline germanium can be performedand used in the formation of nanoclusters 520.

Embodiments of the present invention are not limited to using silicon orgermanium nanoclusters. As will be discussed more fully below, thenanocluster layer is used as a micro-masking layer for an etch that hasthe purpose of adding surface roughness to the sacrificial layer. Anysuitable material that can form nanoclusters that will adhere to thesurface of the sacrificial layer for etching is within the scope of thepresent invention. If the nanocluster material is not compatible withfurther processing, then that material will be removed selectively fromthe sacrificial material subsequent to deposition of a secondpolysilicon layer on the sacrificial layer.

Further, a variety of methods for depositing nanocrystals ornanoclusters can be used (e.g., aerosol coating, spin on coating, andlaser ablation and re-deposition). In one embodiment, low temperature,low pressure chemical vapor deposition is preferred because such adeposition technique fits well within a standard process flow forformation of MEMS devices.

As stated above, one goal is to decrease the surface area of the travelstop that comes in contact with the travel stop region. Thus, thenanoclusters formed on the sacrificial layer should be of sufficientsize to effectively increase the roughness of the travel stop surface.In a typical MEMS device, surface roughness of a polysilicon layerformed on top of the sacrificial layer is about 5 nm. To increase theroughness by five to ten times, nanoclusters should be formed that areon the order of 200 Å. Combined with subsequent processing, discussedmore fully below, nanoclusters of this size provide the desired surfaceroughness. Desired nanocluster size can be provided by selecting processconditions (e.g., deposition time). In addition, the nanoclustersmaterial need not form isolated nanoclusters. Instead, the nanoclusterslayer can be a network of the nanoclusters material that is configuredto allow the underlying sacrificial material to be exposed to etchantfor formation of a topography thereon. In one embodiment, the surfacecoverage of the material can be less than 90%, such that significantroughness can be introduced through the etch.

FIG. 6 is a simplified block diagram illustrating a cross-section viewof the travel stop region during a stage of fabrication subsequent tothat illustrated in FIG. 5. An etch is performed that is selective tothe sacrificial layer. During the etch, the nanoclusters function as amicro-masking layer and indentations 610 are formed in the surface ofsacrificial layer 220. As will be discussed more fully below,indentations 610 serve to increase the surface roughness of travel stop195. For example, in order to form indentations that have a curvedfeature, as shown, a wet etch process is preferred. The length of timeduring which the wet etch is performed should be sufficient to provideindentations of a depth that will aid in formation of a sufficientlyrough surface on the travel stop. Other types of etches, such as ananisotropic dry etch, will form indentations of a different shape.

Wet etch chemistries are dependent upon the nature of the materialsbeing etched and those for which it is desired not to etch (e.g., thenanoclusters). For example, silicon nanoclusters will not etch in aperoxide etchant, while germanium nanoclusters etch readily in aperoxide etchant. Further, the nature of the materials used for thesacrificial layer can dictate the type of etchant materials used.

FIG. 7 is a simplified block diagram illustrating a cross-section viewof the travel stop region during a stage of fabrication subsequent tothat illustrated in FIG. 6. In FIG. 7, nanoclusters 520 have beenremoved from the surface of sacrificial layer 220. This removal of thenanoclusters results in leaving only indentations 610. Removal of thenanoclusters can be performed using an etchant that is selective to thenanocluster material. Alternatively, if the nanocluster material iscompatible with the material to be deposited on top of the sacrificiallayer that will form, for example, the travel stop of a pivoting proofmass (e.g., polysilicon), then the nanoclusters need not be removed asthey will merge with the polysilicon. Thus, silicon-based nanoclusterscan be left on the surface of sacrificial layer if the pivoting proofmass is polysilicon, while germanium-based nanoclusters should beremoved.

FIG. 8 is a simplified block diagram illustrating a cross-section viewof the travel stop region during a stage of fabrication subsequent tothat illustrated in FIG. 7. In FIG. 8, a second polysilicon layer 810 isdeposited over the indented surface of sacrificial layer 220. Thematerial of polysilicon layer 810 fills indentations 610 and forms, forexample, the pivoting proof mass of a teeter-totter accelerometer. Asstated above, formation of polysilicon layer 810 can be performed usingstandard techniques known in the art of forming MEMS devices.

FIG. 9 is a simplified block diagram illustrating a cross-section viewof the travel stop region subsequent to removal of sacrificial layer 220to free the movable portion of the MEMS device, where the travel stop isformed in accord with embodiments of the present invention. Using theprocess described above, travel stop 930 on pivoting proof mass 920 andincludes a surface having bumps 910. In a situation in which travel stop930 comes in contact with travel stop region 145, bumps 910 reduce thecontacting surface area. This reduction in surface area decreases thechance for stiction by both wet stiction and electrostatic/Van der Waalstype forces.

One advantage of reducing the chances for stiction to occur inaccelerometer-type MEMS devices is improved sensitivity of the device.In one type of traditional MEMS accelerometer, stiction force iscountered by increasing a spring constant of the device. But increasingthe spring constant decreases sensitivity of the MEMS device to lightacceleration forces. In another type of traditional MEMS device, chancesfor stiction occurring are sought to be reduced by increasing thedistance between the movable portions of the device and the fixedportions of the device. But this increases the distance between thecapacitive plates and can therefore decrease differences in measuredcapacitance. Reducing stiction forces by using embodiments of thepresent invention allow for lower spring constants and smaller distancesbetween parts, both of which can improve device sensitivity. Further,smaller overall device sizes can be realized by decreasing the distancesbetween the parts. This can, in turn, provide a decreased footprint foreach MEMS device, thereby allowing for incorporation of more MEMSdevices in a system or a smaller system size.

By now it should be appreciated that there has been provided a methodfor manufacturing a micro electromechanical systems device, in which themethod includes: forming a first polysilicon layer over a substrate;forming a sacrificial layer over the first polysilicon layer; forming aplurality of nanoclusters on the sacrificial layer; etching thesacrificial layer using a wet etch process subsequent to forming theplurality of nanoclusters; and forming a second polysilicon layer on thesacrificial layer subsequent to said etching of the sacrificial layer.The wet etch is selective to the sacrificial layer. The nanoclustersprovide a micro-masking layer for the etching, which increases roughnessof the surface of the sacrificial layer as compared to the roughness ofthe surface upon forming the sacrificial layer.

In one aspect of the above embodiment, the method further includesremoving the nanoclusters from the surface of the sacrificial layer.Removing of the nanoclusters is performed subsequent to the etching andprior to forming the second polysilicon layer. In a further aspect, thenanoclusters include germanium and the removing is performed using aperoxide etch. In another aspect of the above embodiment, thenanoclusters include one of silicon or germanium.

In another aspect of the above embodiment, forming the plurality ofnanoclusters includes performing a low-temperature deposition ofnanocluster material and performing an anneal to form the nanoclusters.In a further aspect, performing the low-temperature deposition ofnanocluster material further includes depositing sufficient material toform nanoclusters having a diameter of approximately 20 nm or greaterupon performing said annealing.

Another aspect of the above embodiment further includes forming a firstinsulating layer over the substrate where the first polysilicon layer isformed over the first insulating layer, and forming a second insulatinglayer over at least a portion of the first polysilicon layer where thesacrificial layer is further formed over the second insulating layer. Inanother aspect of the above embodiment, the method further includesremoving the sacrificial layer using a wet etch process subsequent toforming the second polysilicon layer. In a further aspect, the secondpolysilicon layer includes a plurality of surface roughness featureshaving a height between about 25 nm to about 50 nm.

Another embodiment of the present invention provides a microelectromechanical systems device that includes a fixed surface having afirst polysilicon layer formed over a substrate and a first insulatinglayer formed over at least a portion of the first polysilicon layer, anda movable body including a second polysilicon layer providing a majorsurface facing the fixed surface where the major surface includes aplurality of surface roughness features having a height of between about25 nm to about 50 nm.

In one aspect of the above embodiment the movable body includes apivoting proof mass of a teeter-totter accelerometer and a travel stopfeature. The travel stop feature is configured to contact the firstpolysilicon layer to prevent over rotation of the pivoting proof massand includes a portion of the major surface facing the fixed surface.

In another aspect of the above embodiment the movable body is formedusing a process that includes: forming a sacrificial layer over thefixed surface; forming a plurality of nanoclusters on the surface of thesacrificial layer facing away from the fixed surface; etching thesacrificial layer using a wet etch process subsequent to forming theplurality of nanoclusters; and forming the second polysilicon layer onthe sacrificial layer subsequent to etching the sacrificial layer. Thewet etch is selective to the sacrificial layer. The nanoclusters providea micro-masking layer for the etching, and the etching provides aroughness to the sacrificial layer that is imparted to the secondpolysilicon layer during formation of the second polysilicon layer. In afurther aspect, the nanoclusters include one of silicon or germanium. Instill a further aspect, the second polysilicon layer includes at least aportion of the plurality of nanoclusters, if the nanoclusters includesilicon. In another further aspect, the sacrificial layer includes oneof tetraethyl orthosilicate-based silicon oxide or phosphosilicateglass. In another aspect of the above embodiment the MEMS deviceincludes at least a portion of an accelerometer.

Another embodiment of the present invention provides a method formanufacturing a micro-electromechanical systems device, wherein themethod includes: forming a fixed surface; forming a movable bodyproviding a major surface facing the fixed surface where at least aportion of the major surface is configured to contact at least a portionof the fixed surface; and forming of the portion of the major surfaceconfigured to contact the portion of the fixed surface such that theportion of the major surface includes a surface roughness at least fivetimes greater than a non-roughened portion of the major surface.

In one aspect of the above embodiment, forming the portion of the firstmajor surface as that portion of the major surface includes a surfaceroughness at least five times greater than a non-roughened portion ofthe major surface involves: forming a sacrificial layer over the fixedsurface; forming a plurality of nanoclusters on the sacrificial layer;etching the sacrificial layer using a wet etch process subsequent toforming the plurality of nanoclusters; and forming the movable body onthe sacrificial layer after etching the sacrificial layer wherein the atleast a portion of the major surface configured to contact at least aportion of the fixed surface is formed in contact with the etchedsacrificial layer. The wet etch is selective to the sacrificial layer.The nanoclusters provide a micro-masking layer for the etching, and theetching increases roughness of the surface of the sacrificial layer ascompared to the roughness of the surface upon forming the sacrificiallayer. In a further aspect, forming the plurality of nanoclustersincludes performing a low temperature deposition of nanoclustermaterial, and performing an anneal to form the nanoclusters. In still afurther aspect, performing the low-temperature deposition of nanoclustermaterial further includes depositing sufficient material to formnanoclusters having a diameter approximately 20 nm or greater uponperforming the annealing. A size of the nanoclusters is proportional toa time period of the low temperature deposition of nanocluster material.

Because the apparatus implementing the present invention is, for themost part, composed of electronic components and circuits known to thoseskilled in the art, circuit details will not be explained in any greaterextent than that considered necessary as illustrated above, for theunderstanding and appreciation of the underlying concepts of the presentinvention and in order not to obfuscate or distract from the teachingsof the present invention.

Moreover, the terms “front,” “back,” “top,” “bottom,” “over,” “under”and the like in the description and in the claims, if any, are used fordescriptive purposes and not necessarily for describing permanentrelative positions. It is understood that the terms so used areinterchangeable under appropriate circumstances such that theembodiments of the invention described herein are, for example, capableof operation in other orientations than those illustrated or otherwisedescribed herein.

It is to be understood that the architectures depicted herein are merelyexemplary, and that in fact many other architectures can be implementedwhich achieve the same functionality. In an abstract, but still definitesense, any arrangement of components to achieve the same functionalityis effectively “associated” such that the desired functionality isachieved. Hence, any two components herein combined to achieve aparticular functionality can be seen as “associated with” each othersuch that the desired functionality is achieved, irrespective ofarchitectures or intermedial components. Likewise, any two components soassociated can also be viewed as being “operably connected,” or“operably coupled,” to each other to achieve the desired functionality.

Furthermore, those skilled in the art will recognize that boundariesbetween the functionality of the above described operations merelyillustrative. The functionality of multiple operations may be combinedinto a single operation, and/or the functionality of a single operationmay be distributed in additional operations. Moreover, alternativeembodiments may include multiple instances of a particular operation,and the order of operations may be altered in various other embodiments.

Although the invention is described herein with reference to specificembodiments, various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. For example, the description of embodiments of theinvention relates to a teeter-totter type accelerometer. Embodiments ofthe present invention are not limited to teeter-totter accelerometers,but can include accelerometers having a mass suspended by springs, orother MEMS devices that have a potential for components to come incontact with one another during operation or manufacture. Accordingly,the specification and figures are to be regarded in an illustrativerather than a restrictive sense, and all such modifications are intendedto be included within the scope of the present invention. Any benefits,advantages, or solutions to problems that are described herein withregard to specific embodiments are not intended to be construed as acritical, required, or essential feature or element of any or all theclaims.

The term “coupled,” as used herein, is not intended to be limited to adirect coupling or a mechanical coupling.

Furthermore, the terms “a” or “an,” as used herein, are defined as oneor more than one. Also, the use of introductory phrases such as “atleast one” and “one or more” in the claims should not be construed toimply that the introduction of another claim element by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim element to inventions containing only one such element,even when the same claim includes the introductory phrases “one or more”or “at least one” and indefinite articles such as “a” or “an.” The sameholds true for the use of definite articles.

Unless stated otherwise, terms such as “first” and “second” are used toarbitrarily distinguish between the elements such terms describe. Thus,these terms are not necessarily intended to indicate temporal or otherprioritization of such elements.

What is claimed is:
 1. A method for manufacturing amicroelectromechanical systems (MEMS) device, the method comprising:forming a first polysilicon layer over a substrate; forming asacrificial layer over the first polysilicon layer; forming a pluralityof nanoclusters on the sacrificial layer; etching the sacrificial layerusing a wet etch process subsequent to said forming the plurality ofnanoclusters, wherein the wet etch is selective to the sacrificiallayer, the nanoclusters provide a micro-masking layer for said etching,and said etching increases roughness of the surface of the sacrificiallayer as compared to the roughness of the surface upon said forming thesacrificial layer; removing the nanoclusters from the surface of thesacrificial layer; and forming a second polysilicon layer on thesacrificial layer subsequent to said etching the sacrificial layer,wherein said removing is performed subsequent to said etching and priorto said forming the second polysilicon layer.
 2. The method of claim 1,wherein the nanoclusters comprise germanium, and said removing isperformed using a peroxide etch.
 3. The method of claim 1 wherein saidnanoclusters comprise one of silicon or germanium.
 4. The method ofclaim 1 wherein said forming the plurality of nanoclusters comprises:performing a low temperature deposition of nanocluster material; andperforming an anneal to form the nanoclusters.
 5. The method of claim 4wherein said performing the low temperature deposition of nanoclustermaterial further comprises: depositing sufficient material to formnanoclusters having a diameter of approximately 20 nm or greater uponperforming said annealing.
 6. The method of claim 1 further comprising:forming a first insulating layer over the substrate, wherein the firstpolysilicon layer is formed over the first insulating layer; forming asecond insulating layer over at least a portion of the first polysiliconlayer, wherein the sacrificial layer is further formed over the secondinsulating layer.
 7. The method of claim 1 further comprising: removingthe sacrificial layer using a wet etch process subsequent to saidforming the second polysilicon layer.
 8. The method of claim 7, whereinthe second polysilicon layer comprises a plurality of surface roughnessfeatures having a height of between about 25 nm to about 50 nm.